The typical factory gas block is pinned onto the barrel in front of the polymer handguard as part of the front sight assembly. If your rifle features a free-float aluminum handguard, then the gas block is typically a low-profile type that fits under that handguard. The job of the gas block is to take some of the hot gas from behind the bullet that comes through a gas port in the barrel, and direct it into the gas tube which drives the bolt carrier and cycles the action.
Most gas blocks on factory AR-15s are fixed, meaning they just provide a path for the gas from the barrel to the gas tube. With a fixed gas block, all the gas and pressure that comes out of the gas port in the barrel will be used to cycle the action.
In most AR-type rifles there is intentionally more gas than needed, so that if the gun gets dirty, it will continue to cycle.
But often this overgassed situation is more than is really needed, adding to the recoil and increasing wear on parts. That’s where adjustable gas blocks come in. They provide a way to cut off a portion of the gas flow so the action can be driven less forcefully.

The concept of an adjustable-gas system has been around for quite a while. Competition shooters using M1 Garand rifles, a semiauto first used in the US military in 1936, found utility in adjustable gas plugs to regulate the cycling of the action by letting out a little extra gas.
Perhaps the most prolific use was in theFN-FAL (Fabrique Nationale-Fusil Automatique Légerseries, a Belgium manufacturer) rifles developed after World War II. On the FNs, the amount of gas released out of the gas cylinder as the piston was cycling could be adjusted by hand, which controlled how much gas pressure was applied to the piston.
This is similar to what the M1 competition shooters were doing with their adjustable gas plugs.
The normal procedure is to tune down the gas pressure until the gun doesn’t fully cycle, then tune it up one click at a time until it cycles reliably, then go a few clicks further for reliability and you’ll be in the optimal zone. This way the gun is reliable with the least amount of recoil and stress possible, and it can be tuned to a specific type of ammunition as well.

Gas Blocks Today

The JP Adjustable Gas Block (model JPGS-5B) on a barrel.

This same concept is now being applied to the AR-15 and AR-10 family of rifles with modern adjustable gas blocks. The difference is on an AR-15 the gas adjustment cuts off some of the gas flow coming from the barrel instead of letting more of it out after it passes through the gas block, as on the FN-FAL and M1. Early adjustable gas blocks on AR-15s simply used a small metering screw on the side of the gas block that cut off gas flow as you screwed it in. The further you screwed it in, the less gas flow there was.
Once adjusted, the screws were kept in place either by using Loctite or by just letting some carbon build up on the screw to hold it in place. This worked pretty well in practice and has been used by AR-15, 3-Gun shooters for years, but didn’t gain wide acceptance. In the past few years new designs have emerged that have taken the concept to the next level and improved shooter appeal.
Brass locking screws have been added to ensure adjustments stay put without using Loctite, and low-profile versions have been made so they can fit under free-float forearms. Designing them to fit under most of the popular rail systems is a big plus for modern AR builders. Eventually, the desire for lockable settings was solved with spring-loaded detents that were used to lock adjustments in place with audible clicks.
This made it possible to adjust the gas block under the handguard in the field by a known amount and have it automatically lock in place. For those wanting a more mil-spec (military specification or standards) type solution, this was very appealing and has brought these adjustable gas blocks into a far wider acceptance.

The Benefits

You’re probably wondering what difference would it really make in your gun’s performance if you used one of these. Well, there are three:

JP Adjustable Gas Block (JPGS-5B) mounted forward of a free-float JP VTAC Modular hand guard, instead of underneath it, so the gas tube can be seen. The muzzle device is still a PWS FSC-556.

1) You can tune your rifle to run just as hard as is really needed, and by doing so you will experience less recoil and less movement of your sights off target, so your follow-up shots can be faster. What’s happening is that your bolt carrier group will cycle hard enough to extract, eject and feed reliably, but it won’t slam to a stop as hard at the rear of its stroke.

2) There will be less stress on your operating parts because they are not running any harder than necessary. It’s like running your car engine at lower revolutions per minute. Don’t worry, it will not feel slower while shooting.

3) Since you will be cutting off some of the gas going into the bolt carrier group, it’s common to get less carbon build-up in your bolt carrier and on your bolt. This will cause your gun to run cleaner, which can result in better reliability and easier cleaning.

All of these benefits are worthwhile for the shooter who wants to get all the performance possible from his or her rifle.

The SLR Rifleworks adjustable gas block on a barrel.

Installing

So how easy are they to install? If you are familiar with AR builds and installing gas blocks, then installing one of these is the same thing.

For those unfamiliar, it will require removing your flash hider or muzzle brake, removing your handguard, unpinning and removing your factory gas block and installing the new one using set screws or clamping screws to lock it into place. Then you reinstall your handguard and muzzle device. These days, with all the AR-15 home builders out there, this is pretty common knowledge, and the skills required are basic-level gunsmithing.

If you aren’t comfortable with this, find a reputable gunsmith who is proficient in AR builds; this will be an inexpensive gunsmithing job. There is no fitting required; it just involves taking off some parts and then reinstalling some parts. Adjustable gas blocks can be installed on any AR-15 or AR-10 rifle of any caliber or barrel length, as long as it uses a standard gas tube system. ASJ

Editor’s note: Brian Hormberg is the owner of Para Bellum Products, Inc. You can visit the company’s website at nokick.com or follow his blog at ontarget-blog.com.

The Pioneers

By Brian Hormberg

One of the pioneers in the adjustable gas block concept is JP Enterprises, a well-known manufacturer of top-of-the-line race guns for 3-Gun competition. They made an entire system of it by combining adjustable gas blocks with low-mass bolt carriers and low-mass buffers. This concept called for lower reciprocating mass in the moving parts which needed lower gas pressure to run at the right speed. Adjustable gas blocks made this concept possible. The result is even less movement of the gun during firing and even faster recovery from the shot, which is a big advantage in competition. Low-mass systems are normally recommended for competition guns versus duty guns, since a full-mass system can run better when really dirty.

JPmakes their adjustable gas blocks in several formats, including ones with rails on top of the gas block, fixed front-sight models, and low-profile units with lock screws that go under handguards.Syrac Ordnance and SLR Rifleworks are two companies that have recently introduced low-profile, click-adjustable models that can be easily adjusted from the front under the forearm. These are especially well suited to those who want a quick, predictable gas setting change when they switch from subsonic to supersonic or when going from suppressed to unsuppressed. The Syrac Ordnance model is completely self-contained, while the SLR Rifleworks model is designed for easy disassembly and cleaning. Both are small enough to fit under the thinnest aluminum forearm systems.

Another recent trend is to offer this same capability in a piston-driven format in addition to the more common direct gas design. If you like the idea of a piston-driven operating system and like the idea of adjustable gas settings, you can now have both together. The adjustable piston systems can be retrofitted to existing AR-15s or included in new builds just like the other gas blocks and provide the same advantages. The adjustable piston systems include a gas block, gas piston and a complete bolt-carrier group, in addition to the gas block itself to ensure compatibility. Low-profile versions are now in production fromAdams Arms and Syrac Ordnance, and which allow you to fit an adjustable gas piston system under a wider selection of rail forearms than ever before possible and with the adjustment capability.

As more shooters become aware of the advantages of adjustable gas systems, they will likely continue to increase in popularity. All of this is another example of the incredibly wide selection of parts, designs and options available to today’s AR shooter. The operation and performance of the rifle can be customized and tuned to an amazing level, and is only limited by your imagination.

Story by Robin Taylor

Looking down from atop a three-story shooting tower, 12 steel targets stand out along a green hillside, each one further away than the last. They’re all challenging, and the furthest sits at 936 yards.

When the buzzer sounds, you’ll have three minutes to shoot all 12. The problem is, you can’t actually see the targets yet. You’re starting at the bottom of the tower’s stairwell, carrying 200 rounds of ammunition, a coat, a gear bag, a sling, sunscreen, elbow pads, bipod, and a heavy sniper rifle. By the time you get to the top of those stairs and see the targets for the first time, a minute will have disappeared. You’ll be breathing hard, and shooting fast.

“It started out as a way to test the practical use of a precision rifle in a military or law enforcement environment”

This is a Precision Rifle Series match, where extreme accuracy, speed, and physical toughness come together. Sniper matches have been around for a long time, but the PRS is gluing them together into a cohesive, Winston Cup-like string. There’s a $5,000 check at the end for the season points winner, and if you’re the top gun at the PRS National Finale, you could take home a $20,000 purse and prize package, just like last year’s winner, Ryan Kerr of California.

Jim See at the CORE training center in Florida. Note the big pillow-like pad under the foreend, along with the fully adjustable stock and heavy barrel. Items like that pillow pad offer great stability on uneven surfaces like this rockpile. (MICHAEL CAGE PHOTOGRAPHY)

Unlike classic long-range events, PRS has a hard edge – like maybe a 3-Gun competition for sniper rifles. The organizers (notably Rich Emmons) drew ideas from 3-Gun Nation, USPSA/IPSC, and the Bianchi Cup. The result appeals to practical riflemen everywhere.

“It started out as a way to test the practical use of a precision rifle in a military or law enforcement environment,” says Chris Reid at Benchmark Barrels. “From there it’s morphed into a kind of timed field shooting.”

At every match the courses change. The distances aren’t marked, and some of the targets move. Virtually everyone uses a detachable box magazine or DBM in a bolt-action rifle. Mounted to a fiberglass stock or a chassis system, the DBM allows for fast reloading of 10-round magazines. Although shooting a semi auto sounds tempting, experts say the bolt-action rifles with DBMs are more stable in recoil. This platform helps the shooter watch bullet trace and impacts. Seeing the hit or miss guides the shooter to the proper aim for the next shot. Most of the top shooters use 6mm to 6.5mm cartridges, which aid in viewing impacts. The 6mm Creedmoor, 6.5×47 Lapua, and the 6.5 Creedmoor are popular choices, but cartridges up to the .300 Winchester Magnum can be used. Most guns are heavy, but remember, you’ve got to carry it all day – up to 12 hours at a pop. You also carry everything else you’ll need to complete the event, just like you would if you were going afield. There is no going back to the car to resupply – it’s just you and your kit, dealing with changing weather, wind, and lighting conditions.

Jim See firing one of Surgeon Rifles’ guns at the Maricopa County Sheriff’s Range in Arizona. Jim campaigned with a rifle in 6XC in his first season, a caliber that has been gaining support in PRS ever since. (COURTESY OF CHRIS REID)

Jim See shooting a Surgeon Scalpel rifle in 6.5×47 Lapua. This video is from the 2015 Bushnell Brawl held at Rifles Only in Kingsville, Texas. This is the 500-yard mover stage, a target about the size of a football is traveling approximately 3 mph along a track about 60 feet long. At each end it reverses direction, (hence the pause in shooting in the video) Jim hit 7 out of 10 shots in that video which was the high score on that stage.

Reid helps run matches in the state of Washington. The hikes from position to position are arduous enough that out-of-shape shooters won’t finish.

“I’ve seen guys hang it up halfway through,” says Reid.

Short sprints are common in PRS, forcing you to balance the speed advantage of running against how out-of-breath you’ll be when you get there.

More than the sum of its parts

Like the original 3-Gun Nation series, PRS grouped together existing freelance events to make a larger contest. Each event has its own history and traditions, and a different local crew gives each its own special flavor; for example, some require pistol shooting. Scoring varies slightly, but course design varies a lot.

Pay close attention to the course descriptions, because sometimes you can make up a miss for partial credit, and other times, missing wipes out your entire score. If you’ve shot a little long-range, or you’re into long-range hunting, you’ve already got most of the gear.

Jim See, who currently shoots for Team Surgeon Rifles, was building custom rifles in his own shop, Center Shot Rifles, when he first heard about the PRS series. He was “a rifle guy” but didn’t have much experience with practical long-range rifle. The PRS series had just started the year before. “In 2012 I was busy raising kids and stuff, but I managed to place fourth at my first match,” he says. “That’s not the norm, but it shows you that it’s actually pretty easy to get oriented once you get started … I was hooked!”

“I’ve seen guys hang it up halfway through”

Thanks to his day job, See rolled up to the line with an unusually good kit – a Surgeon Rifles action on a McMillan A3-5 stock, in 6mmXC.

“That was a gun I had in the shop,” he says.

See’s friends pushed him to try to make the national PRS Finale, so he went for it, eventually placing 13th in the 2012 series. See won the 2015 Bushnell Brawl this year, making him one of the top guns in the sport. “I was 41 when I started, but I had a lot of experience in various kinds of shooting. If you’ve got some experience in long range, you’ll transition pretty easy.”

Practical Application

“If you take a guy who’s a hunter and have him shoot PRS matches all year, he’ll be able to kill game out to 1,000 yards the following year,” says Reid. “The knowledge and the practicality of it is huge.” If you’re thinking “this isn’t for me,” you might be surprised. Hunters and 3-Gunners deal with unusual firing positions all the time. NRA Bullseye guys have the long-range part down, but often lack the flexibility that practical shooters take for granted.

Awkward terrain forms a big challenge in PRS shooting. Chris Reid tried more-conventional-looking postures, but just couldn’t get settled on this rock pile/shooting position — until he tried laying back. Thank goodness he had a relatively low-recoiling rifle! “I shot that way on the mover too,” says Reid. The rifle is a Benchmark Barrels-built 6.5 Creedmore. Reid runs a suppressor, which helps dampen both blast and felt recoil. (COURTESY OF CHRIS REID)

“An F-class high-master will do great until they have to get into an unusual, nonstandard position,” says Reid. “Without the ability to go prone, they struggle.” People like Shawn Carlock, owner of Defensive Edge, teach long-range hunting classes all over the country, passing on techniques that PRS’ers use. You’ll face the same challenges and more at each and every PRS regional. For someone interested in practical-rifle work, I can’t think of a better training lab than what John Gangl at JP Rifles calls “the anvil of competition.”

“You’re shooting strong-side, weak-side, doing dot drills, moving into and out of positions, and every shot counts,” says Reid.

“This year we have 400-plus guys actively participating in the Precision Rifle Series as competitors,” says See. “These matches cannot be run effectively without dedicated range officers.” ROs set the pace of the match and ensure all participants are safe and receive the points they earned with hits. “It’s nice to travel the country and have fellow competitors volunteer to be range officers on their home ranges. Quality ROs are critical for a successful match,” added See.

A slick member website lays out everything you’ll really need to know, including the dates and locations of all the regional shoots. You can visit them at precisionrifleseries.com. ASJ

Long distances help make PRS matches distinct. Here a shooter reaches out over the plains at Vantage, Wash. (COURTESY OF CHRIS REID)

Ready to compete?, here are some dates to mark, Yes you can click on the map to get more information.